Forensic watermarking for the identification of specific versions of video content is known in the art. According to one method for forensic watermarking, primary and secondary video program identifiers (PIDs) are used at a video broadcast headend to facilitate the insertion of forensic watermarks in an encoded video content item. Typically, most of the original video stream before watermark embedding is broadcast as a “primary video PID”. At least two varying versions of watermarks are generated based on each of a series of small portions removed from the primary video PID. The series of at least two alternate watermarked portions of the video content item is broadcast using the secondary video PIDs. A high number of different forensic watermarks may therefore be effectively created by interleaving among the alternate watermarked versions according to some scheme (such as, for example, according to the key management of cryptoperiods of a conditional access system), such that a final forensic watermark is formed by a concatenation of the interleaved watermarked versions in the rendered content.
Generating the watermarks by removing portions of the primary video PID and preparing corresponding alternate versions for the secondary video PIDs to be added into the stream is generally impractical to perform post-multiplexing, due to the size of a multiplexed video stream (which typically includes other PIDs as well) and the fact that the multiplexed video stream is often scrambled. Accordingly, the portions of the primary video PID to be watermarked are typically replaced by data packets from the secondary video PID after encoding, and before multiplexing. The data packets from the primary and secondary video PIDs are then multiplexed and broadcast to receiving devices for rendering. The rendering device (e.g., set-top box (STB), smart TV, personal computer, etc.), inserts one of the alternate versions of watermarks into each of a series of insertion points in the primary PID stream as the primary PID stream is rendered.
The specific alternate version of a watermark to be embedded is determined according to one or more decryption keys on the rendering device. For example, the packets of the secondary video PIDs may be encrypted using a series of encryption keys, such that different decryption keys are used to decrypt each of the secondary video PIDs. The series of embedded alternate watermarks therefore forms a forensic watermark according to the specific order of decryption keys used to decrypt the watermarked portions, where the specific order itself is a function unique to the rendering device, e.g., the ID of a set-top box. The forensic watermark may then be used to identify the rendering device for a given item of rendered video content.